DANBURY — The state Department of Transportation is pushing back its massive plan to widen Interstate 84 in Danbury.

Construction is now slated to begin in 2023, a year later than originally planned, as officials wade through the earliest planning phases of the $715 million project to widen the highway from Exit 3 to Exit 8.

The project’s completion date is now pushed back to 2030.

“It is important to note that for such a large project, it is difficult to predict the construction dates when we are at such an early stage of the job right now,” DOT spokesman Judd Everhart said.

The dates will remain fluid as engineers prepare a so-called “needs and deficiencies” study of the congested stretch of highway and a federally required environmental assessment of the project. The environmental assessment alone is expected to take until mid-2020 to complete, Everhart said.

“We have not begun the ‘alternative analysis’ yet,” he said. “We do not know what we are building at this time.”

Gov. Dannel P. Malloy first announced the widening plan from a hillside next to the interstate in early 2015 as part of his ambitious 30-year, $100-billion plan to upgrade and modernize transportation infrastructure across the state.

The project has been in its initial planning stages since and DOT officials held a series of workshops last year to gain public feedback on the idea.

More than 36 million cars travel the stretch of I-84 between Exit 3 and Exit 8 each year and it is congested for hours each day beyond the typical morning and evening commutes.

The next round of public input is expected to come with the completion of the environmental assessment in two years. DOT posts updates about the project online at www.i84danbury.com.